155mm Gun Motor Carriage M121-7

M12: General
Date of first acceptance September 1942 Total acceptances 100
Manufacturer Pressed Steel Car Co. Crew 6 men
M12: Dimensions
Combat weight 59,000lbs
27,000kg		 Height over gun shield 113.5"
288.3cm
Length without gun, without sandshields, spade retracted 265.0"
673.1cm		 Gun overhang forward 1.5"
3.8cm
Width without sandshields 105.3"
267.5cm		 Tread 83"
211cm
Ground clearance 17"
43cm		 Fire height 87"
220cm
Ground pressure, zero penetration 12.1psi
.849kg/cm²
M12: Armament
Type Mount Ammunition Traverse Max elevation rate
155mm Gun M1919, M1917A1, or M1918M1 Mount M4 on rear of chassis 10 rounds 28°
(14° left and right;
manual)		 +30° to -5°
(manual)
Aiming equipment
Panoramic telescope M6 and telescope M53 or M53A1 for gunner
M12: Armor
Assembly
Welding
Hull
Rolled and cast homogeneous steel
Location Thickness Angle from vertical
Upper front 1.0"
2.5cm		 30°
Lower front 2.0"
5.1cm		 0° to 46°
Upper sides .625"
1.59cm		 0°
Lower sides .75"
1.9cm		 0°
Rear .75"
1.9cm		 0°
Top .5"
1.3cm		 90°
Floor .5"
1.3cm		 90°
Gun shield .75"
1.9cm		 ~0°
M12: Automotive
Engine Continental R975 C1; 9 cylinder, 4 cycle, static radial, supercharged gasoline
Horsepower Net: 350@2,400rpm
Gross: 400@2,400rpm		 Torque Net: 800 ft-lb@1,800rpm
Gross: 890 ft-lb@1,800rpm		 Fuel capacity 200gal
757L
Transmission Synchromesh, 5 speeds forward, 1 reverse
Steering Controlled differential, steering levers
Brakes Mechanical, external contracting
M12: Suspension
Type Road wheels Track return rollers
Vertical volute spring 3 bogies/track;
2 wheels/bogie		 1 at rear of each bogie
Drive sprockets Idlers Shock absorbers
13-tooth front drive Adjustable at rear of track None
M12: Track
T48
Outside guide, double pin, chevron, rubber
Width 16.56"
42.06cm		 Pitch 6"
15cm		 Shoes/track 79 Ground contact length 147"
373cm
T49
Outside guide, double pin, parallel bar, steel
Width 16.56"
42.06cm		 Pitch 6"
15cm		 Shoes/track 79 Ground contact length 147"
373cm
T51
Outside guide, double pin, smooth, rubber
Width 16.56"
42.06cm		 Pitch 6"
15cm		 Shoes/track 79 Ground contact length 147"
373cm
T54E1
Outside guide, double pin, chevron, steel
Width 16.56"
42.06cm		 Pitch 6"
15cm		 Shoes/track 79 Ground contact length 147"
373cm
M12: Performance
Max level road speed 21mph sustained
24mph dash
34kph sustained
39kph dash		 Max trench 90"
230cm
Max grade 60% Max vertical obstacle 24"
61cm
Min turning diameter 62'
19m		 Max fording depth 40"
100cm
Cruising range ~140mi, roads
~230km, roads

The 155mm GMC M12 was based on the chassis of the medium tank M3. The turret and superstructure were removed, the engine was moved forward to behind the driving compartment, and the 155mm gun was mounted at the rear of the vehicle. The drivers' compartment was the only part of the vehicle fitted with a roof. M12s were built with the three-piece final drive and differential housing and almost all had heavy-duty suspension bogies with the track return roller to the rear of the bogie. Seventy-four M12s were rebuilt by Baldwin Locomotive Works starting in February 1944, correcting problems with engine vapor lock and modifying the engine exhaust pipes and flame arrestors.

The 155mm guns were virtually identical, and used the same ammunition and propellant. The M1917 was a French gun, the M1918M1 was American, and when the M1917 was fitted with the breech ring of the M1918M1, the new weapon was designated as M1917A1.

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References

  1. Hunnicutt, R.P. Sherman: A History of the American Medium Tank. Navato, CA: Presidio Press, 1994. Reprinted with permission from Sherman, R.P. Hunnicutt ©1994, available from Presidio Press, 505B San Martin Drive, Suite 160, Navato, CA 94945.
  2. TM 9-751 155-mm Gun Motor Carriage M12 and Cargo Carrier M30. Washington, D.C.: War Dept., 28 Jan 1944.
  3. Catalogue of Standard Ordnance Items, second edition 1944, volume I: Tank and Automotive. Washington, D.C.: Office of the Chief of Ordnance, Technical Division, 1 June 1945.
  4. Catalogue of Standard Ordnance Items, volume II. 2nd ed. 1944. Washington, D.C.: Office of the Chief of Ordnance, Technical Division, 1944.
  5. TM 9-2005 Ordnance Material - General, Volume V: Sighting and Fire-Control Equipment - General, Aircraft Cannon. Washington, DC: War Department, January 1942.
  6. Chamberlain, Peter, and Chris Ellis. British and American Tanks of World War Two. Frome, England: Cassell & Co., 2000.
  7. Siemers, Cary. "USA's Self Propelled Guns." World War II Tanks & Vehicles and Advanced Squad Leader. 25 Nov 2000. 30 Jan 2001 <http://www.shadowsfolly.com/WWII/USA/SelfPropelled.htm>.
Last updated 17 Jan 2024.
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© Copyright 2001-24 Chris Conners